The present invention relates to press drives. More particularly, the present invention relates to a single speed press drive using an oil shear clutch/brake unit.
Dry friction clutch/brakes depend upon the rubbing of a dry friction material against dry reaction members to start and stop a press. This causes wearing of both the friction material and the reaction members. It also causes heat in these members. The faster the press operates and the faster the flywheel rotates the greater the wear and heat generated. This generation of wear and heat requires periodic gap adjustment to keep the press operating correctly.
Some clutch/brakes used on presses are mechanically interlocked. Mechanical interlocking of the clutch and brake means that a single piston automatically first releases the brake and then engages the clutch on starting. For stopping, the clutch is first released and then the brake engaged. These mechanically interlocked units have most of the clutch/brake mass mounted on the driveshaft and often represent 80 percent of the total inertia of the press that the clutch/brake must stop and start.
Mechanical interlocking of dry friction clutch/brakes reduces the frequency of necessary gap adjustment but does not eliminate this procedure. Adjustment is necessary when the gap has increased sufficiently that press response is affected.
Press builders introduced low inertia clutch/brake designs in an effort to reduce the start-stop inertia. This type of design requires separate pistons to release the brake and engage the clutch. The start-stop inertia with this type design is still usually 60 percent or more of the total inertia. In order for the press to function correctly, the separate pistons must be properly synchronized to prevent overlap. When the clutch starts engaging before the brake is fully released, or, the brake starts engaging before the clutch is fully released, excessive heat is generated, and wear of the friction material and reaction member is greatly increased. Conversely, if there is too much time between the engage/release of the clutch/brake, drifting occurs resulting in sluggish operation and if the drift is high enough, it results in unsafe operation of the press.
The trip rate or cycle speed of a press equipped with a dry friction clutch/brake is limited because the mass of the unit determines its heat dissipation capacity. If this mass is increased, the inertia that must be started and stopped is increased. These factors define a closed loop from which it is impossible to escape when trying the increase the performance of the system.
Accordingly, what is needed is a press drive system which addresses the problems associated with dry friction clutch/brakes, the high inertia clutch/brake designs and the synchronization of the clutch and brake operation.